Influence of the baroreceptor reflex on the modulation of noradrenaline overflow through prejunctional receptors in the portal vein of freely moving rats

Abstract

1. The effects of alterations in mean arterial blood pressure (MAP), as induced by vasoactive drugs, on heart rate (HR), basal noradrenaline concentration and electrically evoked noradrenaline overflow and on blood flow in the portal vein of freely moving rats, were investigated. 2. By infusion of sodium nitroprusside or phenylephrine (0.5, 1.0 and 2.5 micrograms kg-1 min-1), MAP was altered over a range of 50 to 150 mmHg. The resulting changes in HR showed a sigmoidal relationship with MAP. Noradrenaline overflow increased linearly when MAP was decreased; when MAP was increased, however, noradrenaline levels only decreased to 70% and reached a plateau from 125 mmHg onwards. 3. Nitroprusside (2.5 micrograms kg-1 min-1) and fenoterol (0.25 mg kg-1) decreased MAP to the same extent (-46 mmHg). HR and basal noradrenaline concentration, however, were increased to a higher extent by fenoterol (+192 beats min-1; +373 pg ml-1, respectively) than by nitroprusside (+78 beats min-1; +206 pg ml-1, respectively). Electrically evoked overflow was not changed at all after nitroprusside, whereas fenoterol induced an increase to 206% of control. 4. Phenylephrine (2.5 micrograms kg-1 min-1) and angiotensin II (1 microgram kg-1 min-1) increased MAP to the same extent (to 155 and 161 mmHg, respectively). Basal noradrenaline concentration decreased by 30% after phenylephrine, whereas angiotensin II increased noradrenaline levels to 226% of control. Evoked noradrenaline overflow was not changed after phenylephrine but was increased to 204% of control after angiotensin II.(ABSTRACT TRUNCATED AT 250 WORDS)